1. Field of the Invention
The present invention relates to mills for grinding and separating ash and pyrites from coal. More specifically, the invention relates to such mills, which incorporate systems for cleaning and drying coal as well as grinding it.
2. Related Art
In preparing coal for briquetting or pelletizing, ash materials and moisture interfere with successful bonding of finely ground particles into a cohesive mass that will maintain its integrity with storage and handling.
Clay in the ash attracts and holds water. It generally helps maintain a water content of around 30% by weight. This is undesirable because it lowers the Btu value, resulting in increased shipping costs and less boiler efficiency.
In prior art coal processing technology, coarse dewatering takes place at one site. The coarsely dewatered coal is then transported to a grinding site, thence to a peptizing tank, then on to a dewatering screen process or centrifugal operation, followed by aeration to achieve sufficient dryness for maximum bonding results. No ash removal takes place in such prior art technology, unless it is done by wet methods as a separate step.
In addition, in some parts of the wold, coal has extremely high ash content, as much as 35 percent. Currently, conventional milling is done at high maintenance expense to both mills and boilers. Boilers in particular require costly de-slagging to remove encrustations from molten ash. Milling followed immediately, in the same rotating device, by ash separation provides a compact, integrated unit for replacing existing conventional mills as a way to substantially reduce the costs of combusting high-ash coals.
The objects of this invention are several: to dewater incoming wet coal; to fine-grind the coal; to remove minerals and clay components of ash so that the powdered coal can be more thoroughly dried by centrifugal means, down to around 5% by weight, and thus more amenable to briquetting or pelletizing; or to remove ash from coal prior to direct firing.
Experience has shown that by peptizing the clay into a colloidal dispersion by washing the coal particles with vigorous agitation in a soap-like agent, such as sodium hexametaphosphate, the fine platelets of clay can be removed from the small coal particle surfaces making them hydrophobic, thus making it easier to remove the clinging water. The colloidal mix of clay can then be centrifugally separated from the coal. The insoluble mineral portion of the ash can also be centrifuged from the coal. After the peptized clay and mineral containing ash is split off and ejected the clean but still damp coal continues on through an intensive centrifugal drying action.
All of these functionsxe2x80x94coarse coal dewatering, grinding between high speed counter rotating rotors, injection of the peptizing agent into the ground coal stream before it enters the violent agitation zone (where the peptization process takes place), having the treated coal pass through the peptized clay and ash removal point and then on to the final drying phase for the coalxe2x80x94are performed by respective means that are all incorporated into one integral milling unit.
The processor technology preparing coal for pelletizing in accordance with the present invention is not only superior because of what it does and how it does it, but also because of its relative simplicity. There is no coarse dewatering at one site, transporting to a grinding site, thence to a peptizing tank, then on to a dewatering screen process or centrifugal operation, followed by aeration to achieve sufficient dryness for maximum bonding results. The present invention also accomplishes ash removal, which is not achieved in the prior art technology.
Similarly, in preparing high ash coal for direct firing, there is no present technology that performs efficient, high-capacity, ultra-fine milling to liberate high proportions of ash from coal followed immediately by dry separation, nor efficient, stand-alone, high capacity, dry ash separation means.
Other objects, features and advantages of the present invention will be apparent to those skilled in the art upon a reading of this specification including the accompanying drawings.